NSTX XP802 review - S.A. Sabbagh S.A. Sabbagh 1, R.E. Bell 2, S. Gerhardt 2, J.E. Menard 2, J.W. Berkery 1, J.M. Bialek 1, D.A. Gates 2, B. LeBlanc 2, F. Levinton 3, K. Tritz 4, H. Yu 3 XP802: Active RWM stabilization system optimization and ITER support Supported by Columbia U Comp-X General Atomics INEL Johns Hopkins U LANL LLNL Lodestar MIT Nova Photonics NYU ORNL PPPL PSI SNL UC Davis UC Irvine UCLA UCSD U Maryland U New Mexico U Rochester U Washington U Wisconsin Culham Sci Ctr Hiroshima U HIST Kyushu Tokai U Niigata U Tsukuba U U Tokyo JAERI Ioffe Inst TRINITI KBSI KAIST ENEA, Frascati CEA, Cadarache IPP, Jülich IPP, Garching U Quebec NSTX Research Team Review April 8th, 2008 Princeton Plasma Physics Laboratory V1.1 1 Department of Applied Physics, Columbia University, New York, NY 2 Plasma Physics Laboratory, Princeton University, Princeton, NJ, USA 3 Nova Photonics, Inc., Princeton, NJ, USA 4 Johns Hopkins University, Baltimore, MD, USA

NSTX XP802 review - S.A. Sabbagh XP802: Active RWM stabilization system optimization and ITER support Goals  Alter active control configuration to achieve reliable RWM stabilization at various plasma rotation,   Upper/lower RWM B r, B p sensors, follow from best CY2007 feedback settings B r sensor feedback provides RFA correction, B p provide RWM stabilization Determine if stable, low   <   i operation exists with feedback turned off If achieved, control system open as a tool for all NSTX XPs as desired  Specific ITER support requests Study effect of applied time delay on feedback (new control system capability) Determine impact of a large toroidal gap on active RWM stabilization (take out one of six control coils) Addresses  Joule milestone, ITER Organization (IO) request, NSTX PAC request  ITPA experiment MDC-2, ITER issue card RWM-1

NSTX XP802 review - S.A. Sabbagh VALEN code reproduces B pu sensor feedback performance New model simulates experiment  Upper B p sensors located as on device  Compensation of control field from sensors  Experimental equilibrium reconstruction (including MSE data)  Proportional gain Growth rate (1/s) NN DCON no-wall limit Experimental  N (control off) (  collapse) With-wall limit active control passive growth active feedback (present) Experimental  N (control on) state space controller reached

NSTX XP802 review - S.A. Sabbagh 45 o (119761) 315 o (119755) 290 o (119764) 225 o (119770) negative feedback response  f NN   /2  ( kHz )  B pu n=1 (G) t (s) feedback on Feedback control current has relative phase  f to measured  B pu  Internal plasma mode seen at  f = 225 o, damped feedback system response Varying relative phase shows positive/negative feedback Phase scan shows superior settings for negative feedback  Agreement between theoretical and experimental feedback behavior VALEN modeled active feedback Growth rate (1/s)  f (deg) Mode locked Mode rotating Passive poloidal

NSTX XP802 review - S.A. Sabbagh Combination of upper/lower B p sensors used to improve control Feedback phase scan using B pu and B pl  Best phase shown 90 o, not optimal configuration Reduction in  B pu n=1 growth rate  Spatial phase offset between upper/lower B p sensor flux can improve feedback further Control using B pu and B pl also reduces  B r  Correlation of  N collapse and  B ru n=1 amplitude  Suggests that feedback on  B r may allow mode control t (s) 275 o (124034) (124032)  f (124608) NN Poloidal  B pu n=1 (G) Radial  B ru n=1 (G) Midplane B r  B r-ext n=1 (G) Feedback I A (kA) I p (MA) (124033) “B r threshold” reduced growth rate feedback on 225 o 150 o 90 o

NSTX XP802 review - S.A. Sabbagh Feedback on B r sensors alone insufficient for control Feedback on  B ru alone  Clear initial drop in  B ru  Continued drop in  B r and increase in  B rl (also  B pu )  leads to  N collapse Feedback on  B ru and  B rl  Controlled, steady  B ru,  B rl  rapid RWM growth, rotation:  N collapse t (s)  B pu n=1 (G) (upper poloidal)  B ru n=1 (G) (upper radial)  B rl n=1 (G) Feedback I A (kA) NN feedback on Want feedback on both  B r and  B p for most reliable control t (s) unstable RWM n = 1 B pu n = 1 phase n = 2 B pu (deg) (G) (lower radial)

NSTX XP802 review - S.A. Sabbagh Rapid braking   q=2 =0 leads to rotating mode,  reduction Evolving braking physics  Non-resonant n = 3 (NTV), then additional braking seen at resonant surfaces  Peaked   profile leads to strong momentum diffusion (due to island growth?)  Minimize this with broad     /2  (kHz) q q = 2 t (s) n = 3 braking NN poloidal  B pu n=1 (G)   /2  (kHz) I p (MA)   /2  (kHz) core (q ~ 2) NTV resonant R (m)  B odd (G)

NSTX XP802 review - S.A. Sabbagh Reliable feedback stabilization, various plasma rotation Approach for optimization  Start from optimal feedback, using correcting n = 3 phase (medium |   |, unique, broad   profile)  Feedback using all B r and B p sensors – determine best feedback phase / gain  Slow reduction of   using n = 3 braking to vary   profile  Maintain high  N >  N nowall NN  B pu n=1 (G)  B ru n=1 (G)   /2  (kHz) I p (MA) t (s) I A (kA)

NSTX XP802 review - S.A. Sabbagh Goal: create lowest possible   for feedback stabilization Approach reduced rotation from broadest rotation profile possible  Use n = 3 in correcting phasing first, then change to braking phasing  Gate off n = 1 feedback once rotation is slower than typical marginal profiles R (m)   /2  (kHz) t=0.565s (marginal) t=0.996s (stable) t=0.605s (marginal)

NSTX XP802 review - S.A. Sabbagh Several issues need to be addressed for RWM feedback optimization / ITER support Optimization  Feedback with B r sensors to stay below B r n=1 threshold for RWM destabilization  Combined with RWM feedback with upper/lower B p sensors faster control computer for 2008 run  Create low   plasma from broader  , n = 3 corrected target ITER support  Demonstrate RWM feedback control with one coil removed to simulate ITER port plug RWM coil design  Initial shots with neon to support RWM SXR tomography (Tritz)

NSTX XP802 review - S.A. Sabbagh XP802: Active RWM Stabilization / ITER Support (I) Task Number of Shots 1) Create target plasma A) Run active feedback in piggyback mode in prior experiments to verify operation- B) 3 NBI,  > 2.2,  N >  N no-wall (control shot) as setup shot (n=3 correction)2 C) moderate n = 3 braking once core   is reduced; generate RWM3 2) Optimize n = 1 feedback sensors at intermediate   A) Upper/lower B r sensor feedback (start with past “best” FB phase; vary phase)4 B) Vary B r gain2 C) Add upper/lower B p sensors to feedback circuit, vary FB and u/l spatial phase6 D) Vary B p gain 2 3) Active RWM stabilization at low   A) vary onset time, ramp rate, magnitude of n = 3 braking4 B) gate off feedback at low   2 4) Reliability testing A) Repeat best low rotation stabilized shot in repeated shots4 (feedback gated off - add neon for SXR tomography) __________________________________________________________ Total: 29

NSTX XP802 review - S.A. Sabbagh XP802: Active RWM Stabilization / ITER Support (II) Task Number of Shots 5) Examine feedback performance vs. feedback system latency A) Increase feedback system latency from optimized settings to find critical latency for mode stabilization4 6) n = 1 RWM stabilization with one RWM coil omitted A) Create low rotation target plasma with “n = 3” braking; generate RWM2 B) As (A), but with neon for SXR tomography3 C) Upper/lower B r sensor feedback; vary phase4 D) Add upper/lower Bp sensor feedback ; vary phase2 E) Vary feedback gain3 __________________________________________________ Total: 18

NSTX XP802 review - S.A. Sabbagh XP802: Active RWM stabilization - Diagnostics Required diagnostics / capabilities  Ability to operate RWM coils with one coil turned off (Part 6 of run)  Internal RWM sensors  CHERS toroidal rotation measurement  Thomson scattering  USXR  MSE  Toroidal Mirnov array / between-shots spectrogram with toroidal mode number analysis  Diamagnetic loop Desired diagnostics  FIReTip  Fast camera